The present invention relates generally to ways to increase mechanical properties of aluminum alloys, including cast aluminum alloys and components made therefrom, through optimization of solution heat treatment, and more particularly to optimizing a non-isothermal solution treatment based on principles of physical metallurgy and computational thermodynamics and kinetics to achieve target material properties with minimum energy consumption and lead or cycle time.
Aluminum alloys in general, and aluminum-silicon based (Al—Si) alloys in particular (examples of which include A356, 319 and A357) are well known in the automotive and related transportation industries for their strength, ductility and ability to be cast at a relatively low cost. Strengthening by age (also known as precipitation) hardening is applicable to alloys in which the solid solubility of at least one alloying element decreases with deceasing temperature, and solution heat treatment is one way to achieve the desired strength of cast aluminum alloys through precipitation hardening. Examples of wrought and cast aluminum alloys where solution heat treatment can be used to increase age-hardenability include those of the 6000, 7000 and 300 series.
Solution heat treatment serves three main purposes: (1) the dissolution of solute elements from the intermetallic phases that will later cause age hardening, (2) the spheroidization of undissolved constituents, and (3) the homogenization of solute concentrations in the material after casting to achieve a desired strength value. In the traditional solution treatment (either batch or continuous processing in hot-air furnace or fluidized bed), the solution treatment cycle is usually a single step process in which the casting is heated up to a specific temperature and then held at the temperature for a specified time.
As will be appreciated by those skilled in the art, there are many methods for applying solution heat treatments to aluminum alloys. One method is to place the materials in a hot-air furnace. Another method uses a fluidized-bed furnace, where the furnace or fluidizing medium is heated from room temperature directly to soaking temperature and then keep at the soaking temperature for the entire solution heat treatment. There are problems with both of these forms of conventional solution treatment processes. Regarding hot-air furnaces, the heat-treating processes take a long time (for example, from 6 to 10 hrs) at a relatively low and constant temperature. Under these conditions, not only more energy is consumed but also the low melting-point equilibrium phases in the aluminum alloys are dissolved slowly due to the low diffusion kinetics. Such slow diffusion is incompatible with efficient, high-speed manufacture of aluminum alloys and parts, components and related devices made therefrom. In addition, the low solubility of solute elements, due to low soaking temperature in the conventional solution treatment limits the potential of subsequent age hardening. As a result, the materials properties, in particular the tensile strengths are usually low.
Regarding the fluidized bed furnace, a fluidized medium is used that is physically similar to an inert liquid, which in turn means that heat transfer to an article is relatively rapid. In either the hot-air furnace or fluidized bed approach, the furnace or fluidizing medium is heated from room temperature directly to soaking temperature and then kept at the soaking temperature for the entire solution heat treatment, as shown in FIGS. 1A for a batch process and FIG. 1B for a continuous process. In a batch type furnace, the furnace starts heating up after the parts are loaded and furnace door is closed. The parts are also stationary in the furnace. By contrast, in the continuous furnace, the parts are loaded from one end and unloaded from other end of the furnace. The parts are also moving slowly inside the furnace. In either case, the soaking takes a long time, consuming significant amounts of energy in the process. Generally, the continuous processing is better for mass production in comparison with batch processing. Likewise, the prolonged solution heat treatment coarsens eutectic particles (for example, silicon), resulting in silicon depletion at the periphery of the dendrites.
Solution heat treatment is one component of a larger strategy for the heat treatment of age-hardenable aluminum alloys normally. In addition to the aforementioned solution treatment of the products or components at a relatively high temperature (but below the melting temperature of the alloy), two additional steps are typically performed. First, rapid cooling (or quenching) in a cold media, such as water, is carried out at a designed temperature (such as room-temperature). Then the product or component is aged by holding them for a period of time at room temperature (also called natural aging) or at an intermediate temperature (known as artificial aging). The quenching action tends to retain the solute elements in a supersaturated solid solution and also to create a supersaturation of vacancies that enhance the diffusion and the dispersion of precipitates. To maximize strength of the alloy, the precipitation of all strengthening phases should be prevented during quenching. Aging (either natural or artificial) creates a controlled dispersion of strengthening precipitates.
One well-known way to heat treat aluminum alloy castings is the T-6 process. As shown in FIG. 2, T-6 generally involves holding the cast part at high temperatures for extended periods of time (typically 12 or more hours), followed by a water quench and extended aging (often called natural aging, and as long as 24 hours or more), after which a second heat treatment at a lower temperature for another extended period of time (for example, about 8 hours) is used. By placing a casting into a furnace or related processing vessel and heating it to this second heat treatment condition, the casting becomes artificially aged, thereby hardening the metal and increasing its strength in less time than it takes for natural aging.
Despite this, there is a desire to develop a solution treatment cycle for aluminum alloys that avoids the aforementioned shortcomings. There is also a desire to maximize the dissolution of strengthening elements in the solution with less time and energy. There is also a desire to develop a solution treatment cycle that is applicable to various aluminum alloys made by various forging, casting, powder metallurgy or other manufacturing processes.